There were various different blues used by NBC subsidiaries:
East Yorkshire used a deep indigo shade... but
East Yorkshire also used a lighter shade known as 'Riviera blue' for dual purpose vehicles.
Sunderland's was 'midnight blue'
Midland used 'Balfour Beatty blue'
Gloucester used 'aircraft blue'
Latterly there was also a light shade of blue ('Cambridge blue') used by Cambus prior to privatisation.
Jones of Aberbeeg initially used their existing blue colour on purchase by NBC, but this was subsequently changed for NBC corporate blue which was a British Standard 'French blue' colour (BS Series 381C.166 is the colour code).
If this is for a model, then of course colour perception will affect how things look due to size, different material being painted, different undercoat, different lacquer coat, etc, etc such that the right shade of blue might actually look wrong to the eye.